/:16 Yesterday Facebook rolled out a new feature called Places that lets you and your friends check in to locations, Foursquare-style. If you'd prefer to keep your location private, or at least stop your friends from posting it, here's how.
UPDATE: This post is a little outdated. For more recent information, visit our Always Up-to-Date Guide to Managing Your Facebook Privacy.
If you're not convinced that posting your location can be a bad thing, check out PleaseRobMe for some evidence. Of course, if you're careful, check-ins aren't inherently a bad thing. Whether or not you want to disable them is entirely up to you, but Facebook—yet again—has made the assumption that you want to take part in all of their privacy-eroding new features. If you don't, or want a little more control over who can divulge your location, you can make this change pretty quickly through your privacy settings.
1) Log in to Facebook. From your Account menu, choose Privacy Settings. You'll get something like the picture below. Click the Customize option (if it isn't already selected) and then click the "Customize settings" link (it's the one next to the pencil below the table depicting your current privacy settings).
2) Under the "Things I Share" heading, all the way at the bottom (of that section), there are two things you may want to change. By default "Places I check in" should be set to only be visible by your friends. If you want to limit it more or less, use the drop down menu to do so. I set mine to "Only Me," which is as private as you're going to get. Below that option is "Include me in 'People Here Now' after I check in." It is enabled by default. This will let people know you're at a particular location via the location's page or in a search for people near you. Uncheck "Enable" if you don't want this.
3) Lastly—and this is the important one—if you don't want your friends to check you into Places, sharing your current location with a bunch of people you may or may not know, go down to the section called "Things Others Share" and find "Friends can check me in to Places." Initially, mine wasn't set to anything at all so the default option could be either choice. Regardless, set this to "Disabled" if you don't want your friends checking you in. Keep in mind that any friend could potentially check you in anywhere. You don't actually have to be there. If you don't want anyone playing a practical joke and checking you in to a strip club, for example, this is a good thing to turn off.
UPDATE: As commenter Ryan G. points out, your location can be shared with friends' applications as well. To turn that off, you have to go to a completely different section. Click the "Applications and Websites" link (pictured to the left) and then go to the "Info accessible through your friends" section. Click "Edit Settings" and you'll see a bunch of boxes. Anything checked is available to Facebook apps that your friends are using. The last checkbox in this section is "Places I've visited," so uncheck that if you don't want your friends' apps grabbing your location information as well.
Since this part is confusing a lot of people, here's a quick video to demonstrate how to edit your application settings:
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And that's it! While it's not so great that you're opted-in to the new Places feature, fortunately it's pretty easy to opt-out.